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52 rule of thumb for a microsurfacing is that it can carry erage. Clean and lightly dampen the area to be hand-worked traffic when it is expelling clear water (National High- before placing the mix. Ensure areas that require handwork produce a finished surface that is uniform in texture, dense, way Institute 2007). and has a neat appearance similar to that produced by the · Streaking: Streaking is caused by one of two condi- spreader box. Microsurfacing material required to repair defi- tions during construction. Insufficient embedment allows ciencies due to unsatisfactory workmanship and the work larger stones to be caught by the strike-off rubber and required to mix and place the materials according to the Spec- ifications will be provided at no expense to the Department dragged along the surface. Excess build-up of material (Georgia DOT 2001). in the spreader box has the same effect (ISSA 2010a). · Delaminating: The major cause of delamination is fail- This discussion shows agreement between the literature and ure to properly prepare the surface before commencing the specification content analysis and leads to the following microsurfacing. It can also be caused by the emulsion effective practice: breaking too fast, keeping the bond with the substrate from forming (Smith and Beatty 1999; Austroads 2003b). Holding a pre-paving meeting to discuss quality manage- ment and workmanship issues before full production micro- Example Microsurfacing Quality surfacing provides a forum where both the agency and the Assurance Specification contractor can address main areas and concerns about microsurfacing quality. The Georgia DOT microsurfacing specification (2001) includes a section specifically titled "workmanship" that speaks MICROSURFACING PERFORMANCE to the concerns discussed earlier and makes them enforceable contract requirements. The specification is included here as an The purpose of any quality management program is to not example of how to articulate these concerns in the contract. only ensure that the product meets the contract requirements but also to ensure that the product is constructed in a manner Workmanship--Excessive buildup, uncovered areas, or unsightly appearance are not permitted on longitudinal or transverse joints. that permits it to perform as designed. Therefore, connecting Place longitudinal joints on lane lines. Excessive overlap is not the quality management practice discussed previously with permitted. Ensure straight lines along the roadway centerline, agency information about microsurfacing performance allows lane lines, shoulder, or edge lines. Keep lines at intersections the analyst to draw inferences about the effectiveness of dif- straight to provide a neat and uniform appearance. ferent approaches to this critically important topic. 1. Finished Surface: Ensure that the finished micro-surfacing has a uniform texture free of excessive scratch marks, tears, or other surface irregularities. Excessive tear marks are considered Definition of Microsurfacing Success 4 marks that are 1/2 inch (13 mm) wide or wider and 6 inches (150 mm) or more long per 100 square yards (85 meters), or Just as the term "quality" has many different definitions that any marks 1 inch (25 mm) wide or wider or 4 inches (100 mm) long. Ensure that the edges of the micro-surfacing appear neat depend on who and what it is related to, agency definitions of and that longitudinal alignment is parallel to the roadway microsurfacing performance vary among the agencies them- centerline. selves. The survey sought to draw out those answers and aggre- 2. Joints and Seams: Produce neat and uniform longitudinal gate them to develop a rank ordering of 5+ standard definitions and transverse joints. Construct transverse joints as butt-type joints. Place longitudinal joints on lane lines when possible. from the literature to identify any trends that might be present Do not allow gaps between applications. Joints are acceptable in the data. Table 35 shows the results of that analysis, with if there is no more than a 1/2 inch (13 mm) vertical space for an interesting trend evident. In chapter three, the reasons why longitudinal joints nor more than 1/4 inch (6 mm) for a trans- agencies select microsurfacing for a given pavement mainte- verse joint between the pavement surface and a 4 ft (1.2 m) straightedge placed perpendicular on the joint. nance/preservation project were covered and a number of agen- 3. Areas the Mixing Machine Cannot Reach: Surface these cies added the comment that their purpose for using microsur- areas using hand tools to provide complete and uniform cov- facing was to extend the life of the underlying pavement. This TABLE 35 SUMMARY OF AGENCY DEFINITIONS OF MICROSURFACING SUCCESS Test U.S. Canada Total Meets expected service life 19 6 25 Meets project specification requirements 14 6 20 Qualitative measures--look, color, etc. 8 8 16 Does not fail shortly after construction 7 5 12 Achieves desired friction/skid number 9 1 10 Meets texture standard (>0.6 mm) 1 0 1 No maintenance expenditures over life 1 0 1 Note: Agencies were asked to check all that applied.
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53 attitude is validated by the most frequently cited success met- · The application rate was too thin to hold larger aggregates, ric; meets expected microsurfacing service life. That leads to · The matrix has a lack of fines to fill voids between larger aggregates, the conclusion that microsurfacing is viewed as a valuable · Cooler temperatures may result in slowing of the cure necessary pavement preservation treatment rather than merely a pave- for traffic, ment maintenance treatment. The second most cited success · Premature opening to traffic, and metric was that the treatment met project specifications. This · Rain fell on the microsurfacing prior to complete setting (ISSA 2010a). probably relates more to administrative process for contract payment than the long-term quality of the microsurfacing itself. Delamination is almost always the result of improper prepa- ration of the substrate surface before microsurfacing (Smith Post-construction visual assessment was the third most and Beatty 1999; Austroads 2003a; ISSA 2010a). It can also be common success metric followed by the absence of short-term caused by the emulsion breaking too quickly, which results in failure. The friction and texture metrics are the only two post- a broken mix being placed on the surface that will not form a construction metrics that can be physically measured. The bond (Austroads 2003b). Finally, improper transverse and lon- performance-based pavement maintenance contracts in use gitudinal joints were also cited as post-construction micro- in New Zealand have as many as 200 post-construction per- surfacing defects. Both of these are workmanship issues. The formance criteria (Manion and Tighe 2007), many of which ISSA manual describes the cause as follows: involve direct measurements of the pavement's surface char- acteristics. This explains why the Austroads respondents did · Transverse: The transverse joint was constructed with- not check qualitative measures as part of their success defini- out using roofing felt, metal strips, etc., at the start of the tion and leads to identification of an area for future research: placement pass. Similarly, the spreader box was either evaluating engineering measurements used by Austroads pulled until empty or ended without stopping on roofing (2003b) as acceptance tests for microsurfacing projects. felt or other protective surface meant to ensure a straight transverse joint. Poor joint construction practices result Minimizing Post-Construction in excessive material build-up, uncovered areas, and un- Microsurfacing Defects sightly appearance. Although proper joint construction techniques are followed, occasionally the mixture may The survey also asked respondents to share the types of dis- not be performing as designed owing to changing envi- tresses that they most often found in their microsurfacing proj- ronmental conditions (ISSA 2010a). ects. Table 36 shows the results of that analysis. One can see · Longitudinal: The placement machine may not have that the most common distress found in microsurfacing was driving controls on both sides of the equipment so that the reflected cracking. This confirms the conclusion drawn in chap- operator can follow existing edge markings, string lines, ter three that microsurfacing is not effective in treating serious and previously placed microsurfacing in adjacent lanes. cracking. The second most common distress was streaking and Many times the problem is related to poor planning of it is directly related to the quality of the workmanship. the product application process by the contractor. Exces- sive buildup, uncovered areas, or unsightly appearance Raveling and delamination are the next two most common often results from poor alignment of the longitudinal distresses in microsurfacing. Raveling can be caused by a joint (ISSA 2010a). number of material, design, or construction quality issues. A list of the most common is as follows: The survey also asked the agency respondents to rate eight preconstruction factors on their ability to minimize these · The aggregate lacks sufficient embedment in the matrix caused from insufficient asphalt quantity to hold the larger aggregate, defects. The results of that analysis are shown in Table 37. · Poor quality aggregates may debond from the matrix, It can be seen that the two different population groups TABLE 36 SUMMARY OF COMMON MICROSURFACING TABLE 37 POST-CONSTRUCTION DISTRESS IMPACT OF PROJECT FACTORS ON MICROSURFACING QUALITY Distress U.S. Canada Total Rated Impact Crack Reflection 15 5 20 (1 = highest rated factor) U.S. Ranking Canadian Ranking Streaking 9 2 11 Contractor Experience 1 2 Raveling 6 4 10 Selecting the Right Project 2 1 Delamination 7 1 8 Construction Procedure 3 3 Transverse Joints 5 3 8 Preconstruction Road Preparation 4 7 Bleeding 4 1 5 Better Aggregates 5 5 Longitudinal Joints 4 0 4 Better Binder 6 6 Corrugation 1 1 2 Design Method 7 4 QC/QA Program 8 8 Note: Agencies were asked to check all that applied.
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54 TABLE 38 REASONS FOR MICROSURFACING FAILURE Cause of Failure U.S. Canada Total Improper application rate 5 5 10 Dirty or dusty aggregate/gradation issues 4 4 8 Wrong road--poor project selection 6 2 8 Improper ambient and/or surface temperatures 3 3 6 Improper binder viscosity 3 3 6 Improper binder temperature 3 3 6 Improper surface preparation 3 2 5 Weather 2 2 4 Field construction procedures 1 0 1 Snow plow damage 1 0 1 cited the same top three factors: contractor experience, The aggregate issues cited in the table could be a function proper project selection, and construction procedures. Cou- of doing gradation testing at the source (see Table 33). The pling contractor experience as having the most impact on aggregate will be handled several more times after it leaves quality with the finding in chapter four that the availability the pit and research has shown that every time an aggregate is of qualified microsurfacing contractors was a major concern handled its gradation changes as the handling of the material leads to the conclusion that a certification program for micro- creates more fines. Although it cannot be determined from surfacing contractors is not only necessary, but it is also an this study, a possible fix would be to sample the aggregate urgent initiative. gradation as close to its introduction into the mixing machine as possible. Thus, an aggregate that is of marginal quality in This analysis also indicates that project selection is probably terms of soundness and abrasion resistance would be tested the most important step in the microsurfacing design process in the final gradation that is incorporated into the microsur- with regard to impact on the final performance of the micro- facing fix. Finally, project selection comes up once more as surfacing itself. Finally, the relative importance of the material a reason for microsurfacing failure, underscoring the impor- quality, design method, and QC/QA program indicates that the tance of that step in the design process. primary focus of the microsurfacing quality management pro- gram needs to be on workmanship rather than materials. By The survey also sampled agency experience regarding com- definition, microsurfacing is designed to incorporate high- plaints from the traveling public and their source. Table 39 quality materials (Johnson et al. 2007; ISSA 2010a). Thus, contains the results. It is gratifying to see that the most com- ensuring that the high-quality material is properly installed mon answer was "we don't get complaints." That statistic leads to the following effective practice: speaks volumes about the ability of microsurfacing to satisfy the needs of the traveling public for a safe, comfortable sur- Focus agency construction quality assurance efforts on face upon which to drive. Road noise and appearance were those microsurfacing factors that relate to the quality of the the next most common public complaints. Both of these are workmanship and other field-related aspects. perceptional and were discussed in chapter three. Microsurfacing Failures Microsurfacing Service Life Despite the best efforts of quality-conscious contractors and Finally, the survey asked the respondents to indicate what fac- agency inspectors, microsurfacing projects do experience fail- tor was most critical to microsurfacing achieving its intended ures. The survey sought to gauge the magnitude and reasons service life (Table 40). The overwhelming answer from both for failures in microsurfacing projects. Table 38 shows the groups was essentially selecting the right project for this results of those data collection efforts. A trend in the failures is evident. The top reason for failure cited was using an improper TABLE 39 application rate. The application rates are developed as part PUBLIC COMPLAINT SUMMARY of the job mix formula development process, but need to be Public Complaints U.S. Canada Total adjusted in the field. Assuming that the majority of the time No Complaints 5 3 8 the contractor is furnishing the job mix formula and has Road Noise 8 1 9 control over the application rates in the field, this failure is the Appearance 5 3 8 contractors' responsibility. This information further validates Loose Stone 1 1 2 the concern expressed in the previous paragraph with the Vehicle Ride 1 0 1 need for qualified and experienced microsurfacing contrac- Do Not Know 8 0 8 tors to promote successful microsurfacing projects.